Competition

Competition can be defined as an interaction between organisms or species, in which the fitness of one is lowered by the presence of another. Limited supply of at least one resource (such as food, water, and territory) used by both usually facilitates this type of interaction, although the competition may also exist over other 'amenities', such as females for reproduction (in case of male organisms of the same species).[13] Competition is one of many interacting biotic and abiotic factors that affect community structure. Competition among members of the same species is known as intraspecific competition, while competition between individuals of different species is known as interspecific competition.
Interspecific competition is normally not as fierce as intraspecific competition, unless in case of a sudden drastic change. However, it is the most conspicuous competition in grasslands, where, for example, cheetahs and hyenas are often killed by lion prides. Competition is not always a straightforward, direct interaction either, and can occur in both a direct and indirect fashion.
Competition between species at the same trophic level of an ecosystem, who have common predators, increases drastically if the frequency of the common predator in the community is decreased by a large margin. The magnitude of competition therefore depends on many factors in the same ecosystem.
According to th

competitive exclusion principle, species less suited to compete for resources should either adapt or die out. According to evolutionary theory, this competition within and between species for resources plays a critical role in natural selection.
Fitness (often denoted in population genetics models) is a central idea in evolutionary theory. It can be defined either with respect to a genotype or to a phenotype in a given environment. In either case, it describes the ability to both survive and reproduce, and is equal to the average contribution to the gene pool of the next generation that is made by an average individual of the specified genotype or phenotype. If differences between alleles of a given gene affect fitness, then the frequencies of the alleles will change over generations; the alleles with higher fitness become more common. This process is called natural selection.
An individual's fitness is manifested through its phenotype. The phenotype is affected by the developmental environment as well as by genes, and the fitness of a given phenotype can be different in different environments. The fitnesses of different individuals with the same genotype are therefore not necessarily equal. However, since the fitness of the genotype is an averaged quantity, it will reflect the reproductive outcomes of all individuals with that genotype in a given environment or set of environments.